C# Render WebControl to string

Add reference to System.Web

StringBuilder sb = new StringBuilder();
System.Web.UI.HtmlTextWriter tw = new System.Web.UI.HtmlTextWriter(new StringWriter(sb));

System.Web.UI.WebControls.DataGrid htmlTable = new System.Web.UI.WebControls.DataGrid();
htmlTable.DataSource = ds.Tables[0];
htmlTable.DataBind();
htmlTable.RenderControl(tw);

MessageBox.Show(sb.ToString());